Diffusional Transformation Kinetics from Work-hardened Austenite

Abstract

The deformation of austenite below its recrystallization temperarure markedly accelerates the subsequent diffusional transformation. This acceleration is mostly caused by the increase in the nucleation rate. The increase in the nucleation rate per unit volume of specimen is attributed (1) to the increase in the austenite grain area by the elongation of grains, (2) to the increase in the nucleation rate per se per unit area of grain surface, and (3) to the formation of additional nucleation sites such as annealing twin boundaries, deformation bands, etc. Considering these effects, a kinetic equation for the deformed sepcimens was derived and from this equation, a method to estimate quantitatively the individual effect of the above three factors on the increase in the transformation rate was developed by defining the acceleration multiplying factors. Furthermore, the effects of the amout of rolling reduction and austenite grain size on the individual effect of the three factors on the acceleration of transformation were discussed based on the derived kinetic equation. Finally, as an application of the developed method, the acceleration multiplying factors for the pearlite transformation in the hot-rolled SKD 6 steel were calculated as functions of rolling reduction and austenite grain size.